The present invention relates to improvements in a lockup control apparatus of a torque converter having a lockup mechanism for establishing a direct connection between input and output elements of the torque converter.
Normally, an automatic transmission is provided with a torque converter for increasing torque and absorbing torque fluctuation. Such a torque converter is generally of a lockup type arranged to directly connect input and output elements of the torque converter under a predetermined condition so as to improve the energy consumption by the improvements of the power transfer efficiency. For example, the lockup of the torque converter is executed in a running condition where it is not necessary to ensure torque increasing function and torque fluctuation absorbing function.
In order to improve the energy consumption by further improving the transfer efficiency of the torque converter, it is preferable to set the torque converter into the lockup engagement condition when the vehicle is put in a coast running where a throttle opening is set at 0/8 as shown in FIG. 14. That is, it is preferable to expand a lockup range under the coast running to a lower load operation and a lower vehicle-speed operation as possible. However, when the vehicle is quickly decelerated by the operation of a brake apparatus during the coast running while being traveling on a low-friction lead, wheels of the vehicle are suddenly stopped. In such sudden stop situation, it is difficult to execute the release of the lockup engagement of the torque converter before the sudden wheel stop so called wheel lock. This invites a trouble such as an engine stall.
Japanese Patent Provisional Publication No. 8-21526 has proposed a control apparatus for solving such a trouble while keeping energy saving. The control apparatus is arranged to calculate a minimum lockup engagement capacity, which is the smallest value where no slip is generated between input and output elements of a torque converter, on the basis of a detection value and estimated value of a reverse drive torque. When the vehicle is put in the coast running, this control apparatus controls the lockup engagement force so as to maintain the lockup capacity at the minimum lockup engagement capacity.
However, the characteristic of the friction coefficient .mu. of the lockup clutch against the slip is generally fluctuated by each individual due to the difference of a facing, temperature and aging. Since this conventional control apparatus is arranged to control the lockup capacity at a predetermined minimum lockup engagement capacity, if the characteristic of the friction coefficient .mu. of the facing of the lockup clutch is dispersed among individuals, it is possible that the slip between the input and output elements of the torque converter will be generated even if the lockup capacity is controlled at the predetermined minimum lockup engagement capacity.